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Behavior of Null Values in Equations (Expressions)

Any arithmetic calculation in VBA with a Null value results
in a null value. If one of the values is NULL, the result is Null (blank).
According to VBA rules for calculating values, the result is null because a
null value is an unknown.

Nulls are not the same value as zero (0). However, in many situations that's exactly what we want. We
want to treat Nulls as zeros.

Thankfully, Microsoft Access offers a way around this issue in VBA module code or queries. The NullToZero function (NZ) takes a null
value and converts it to zero. You should use this function anywhere a value
could be null (like a field that is not required).

Example for Using the NZ Function

The following can be used to calculate the sum of two fields in the query:

Nz([Field1]) + Nz([Field2])

If [Field1] is null and [Field2] is 2, the result is 2.
Without the Nz() function, the result would be null. So be safe! Wrap your
potential null values with NZ.

Syntax for NZ Function

Definition

Nz(Value, ValueIfNull)

Parameters

There are two parameters. The second one is optional and lets you override the
default value for nulls. For instance, you may want Nulls to be treated as 1, or for text
fields, a zero-length string ("") rather than zero.

Parameter

Required or Optional

Data Type

Description

Value

Required

Variant

Value to evaluate for Null

ValueIfNull

Optional

Variant

The custom return value if the value being evaluated is Null (overrides the default value)

Return Value

If the Value parameter is not null, the Value is returned.

If the Value parameter is null:

If the ValueIfNull parameter is provided, ValueIfNull

If the ValueIfNull parameter is not provided, a zero (0) or zero-length string ("").
Note that when used in a query expression, the NZ function returns a zero-length string.

Check out the VBA help file for the complete definition of the NZ function.